During creation of a subsea well, a wellhead assembly including a wellhead housing is located at the upper end of the wellbore at the sea floor. As the well is drilled deeper, a drill string passes through the wellhead housing. One or more casing strings are lowered through the wellhead housing, each supported with a casing hanger that lands in the bore of the wellhead housing. A packoff or casing hanger seal is set in a seal pocket between a side surface of the casing hanger and a sidewall of the bore. The casing hanger seal is preferably a metal-to-metal seal, which bests seals if the sealing surfaces on the casing hanger and on the sidewall of the bore are in good condition.
Wellhead drilling operations may cause damage to the sidewall of the wellhead housing bore before the casing hanger seal is installed. In particular, casing hangers and the high pressure wellhead housing can be damaged with scratches and gouges that range from minor scratches, such as a few thousandths deep, to major scratches, as much 0.1″ deep. To seal a casing hanger annulus that has been damaged, it may be necessary that the seal is constructed of a compliant material that can extrude and fill the scratches and gouges. If the desired metal-to-metal seal is unable to seal adequately, it is normally removed and replaced with an emergency seal. Normally, emergency seals consisting of elastomeric seal elements are used to seal the casing hanger annulus. The emergency seal may also have metal sealing elements combined with the elastomeric element.
Retrieving a primary metal-to-metal seal that fails to meet a pressure test is a time-consuming task. The operator has to release the primary seal from its set condition and retrieve it with a string of drill pipe. The operator then has to run an emergency seal with a running tool on a string of drill pipe. In deep water, the cost to trip a string of drill pipe from the drilling vessel to the subsea wellhead housing is expensive.